Methacrylic resins, those are thermoplastic polymers composed mainly of polymethyl methacrylate are used in various fields due to excellent properties of transparency (high total light transmittance in the visible light region) and surface hardness.
For example, transparent methacrylic resin films are used in the interior of automobiles, exterior of household appliances, wallpapers, and other substrates, for the purpose of decoration or design with wood grain or other patterns; and protections for the substrates such as addition of scratch resistance and weather resistance.
Moreover, methacrylic resin composites (composite films etc.) coated with metal and metal oxide film are used in order to add design such as a metallic pattern to substrates, and to protect substrates (adding scratch resistance and weather resistance).
However, methacrylic resins may lack mechanical properties, particularly toughness, depending on the application. In particular, methacrylic resin films are extremely brittle, making film formation difficult and also causing very poor handleability. Some methacrylic resin films crack when trimmed simultaneously with film formation, cut after film formation, or pasted to a substrate, or when unnecessary portions (burrs) are removed after the film is pasted to the substrate.
Then, in order to remedy brittleness of molded articles composed only of methacrylic resins, acrylic resin compositions containing so-called core-shell particles and molded articles obtained by using such acrylic resin compositions are proposed.
For example, Patent Literature 1 proposes a film- or sheet-like molded article obtained by mixing core-shell particles (commonly known as two-layer core-shell particles) obtained by copolymerizing a methacrylic acid alkyl ester and an acrylic acid alkyl ester in the presence of particles of a cross-linked acrylic acid alkyl ester polymer, with a methacrylic resin.
The acrylic resin molded articles containing such two-layer core-shell particles generally have low surface hardness. For remedying this, so-called three-layer core-shell particles (Patent Literature 2) and acrylic resin molded articles containing such three-layer core-shell particles (Patent Literatures 3 and 4) are proposed.    Patent Literature 1: Japanese Examined Patent Publication No. 56-27378    Patent Literature 2: Japanese Examined Patent Publication No. 55-27576    Patent Literature 3: Japanese Registered Patent No. 3287255    Patent Literature 4 Japanese Registered Patent No. 3287315
However, in molded articles comprising the resin compositions obtained by mixing core-shell particles, although impact resistance is improved, toughness is insufficiently improved. Additionally, rubber components contained in the molded article cause decreases in surface hardness, rigidity, heat resistance, and transparency (particularly increase in haze). Further, when the molded article is stressed, for example, stretched or bent, the stress riser may be whitened. Moreover, when the molded article is subjected to impact or left under heat and humidity conditions for a long time, whitening may occur. Such a whitened molded article loses transparency, and design characteristics and high-grade feeling of the molded article are likely to be impaired.
Further, core-shell particles themselves are cross-linked, and thus have no flowability. Therefore, parts of core-shell particles fatally project from the surface of the molded article. This degraded the surface smoothness of acrylic resin molded articles. Films having reduced surface smoothness are increasingly easy to crack when cut after film formation or pasted to a substrate, or when unnecessary portions (burrs) are removed after the film is pasted to the substrate.
When a layer of metal and/or metal oxide is formed on the surface of an acrylic resin molded article (e.g., a film), the surface smoothness of the molded article has very important influence. That is, the surface smoothness of the molded article directly affects the surface smoothness of the metal and/or metal oxide layer formed on the surface of the molded article, and eventually affects handleability and design characteristics such as specular gloss of the composite.
Further, when the surface of other resin is decorated with an acrylic resin molded article on which a metal and/or metal oxide layer is laid by an injection-molding simultaneous lamination method, low surface smoothness of the acrylic resin molded article causes a dramatic decrease in surface gloss.
Then, as a film that has high surface smoothness and particularly can exhibit good design characteristics even when a layer of metal and/or metal oxide is formed on the film, Patent Literature 5 proposes an acrylic resin film comprising acrylic rubber particles, obtained by melt-extruding an acrylic rubber particle-containing acrylic resin from a T-die, and forming the extruded resin into a film while being sandwiched between two mirror-finished rolls at a pressing pressure (linear pressure) of 300 N/cm or higher, wherein at least one side of the film has a center line average roughness Ra (defined in JIS B0601) of 0.01 to 0.05 μm.
In Patent Literature 5, Ra is measured using a measurement stylus with a tip radius of 3 μm and a surface roughness meter (Surfcom 570 A; manufactured by Tokyo Seimitsu, Co., Ltd.). As a result of investigations, the present inventors found that the size of this measurement stylus is much larger than that of a core-shell particle (usually from several tens to several hundreds nm in size), and therefore the degree of projection of core shell particles etc. could not exactly be evaluated. The inventors also found that the presence of aggregated core-shell particles (usually from several hundreds to 1 μm in size) was not significantly reflected in Ra. Thus, even an acrylic resin film having a center line average roughness Ra in the above range could not sufficiently prevent undesirable cracks originating from defects caused by projected core-shell particles or aggregated core-shell particles when the film was cut after film formation or pasted to a substrate, or when unnecessary portions (burrs) were removed after the film was pasted to the substrate.    Patent Literature 5: Japanese Patent Laid-Open No. 2003-253016
Furthermore, when resin compositions comprising only core-shell particles were stretched, bent, or subjected to impact, the processed parts were easily whitened, thereby reducing transparency, and toughness was insufficient. In addition, resin compositions comprising only core-shell particles were hard to satisfy handleability, surface hardness, and surface smoothness of acrylic resin molded articles (films) at the same time.
As other methods for improving toughness of methacrylic resins, other polymer to be blended in a methacrylic resin is proposed.
For example, polycarbonate is mentioned as a polymer that can be expected to improve balance among toughness, heat resistance, and transparency. A composition of polymethyl methacrylate and polycarbonate of bisphenol A can reportedly be obtained by, for example, dissolving polymethyl methacrylate and polycarbonate in tetrahydrofuran, precipitating the mixture in heptane, and heat-treating the precipitate at a temperature higher than the glass transition temperatures of the polymethyl methacrylate and polycarbonate. However, molded articles formed from this composition have low surface hardness. Further, since a solvent is used for the preparation of the composition, great deal of energy is necessary to remove the solvent. Thus, productivity is low. In addition, a method of melt-kneading polycarbonate and polymethyl methacrylate is also reported. However, compositions obtained by melt-kneading produce opaque and pearlescent molded articles because of phase separation of polycarbonate and polymethyl methacrylate (Non Patent Literature 1).    Non Patent Literature 1: Journal of Polymer Science PART B, Polymer Physics, Vol. 25, 1459 (1987)
Polyvinyl butyral is mentioned as a polymer potentially compatible with polymethyl methacrylate.
Non Patent Literature 2 discloses that since a methyl methacrylate resin and polyvinyl butyral are weakly compatible, products obtained by mixing them usually have a two-phase structure as a result of phase separation; however, a methyl methacrylate resin having a low molecular weight and polyvinyl butyral are possibly compatible, thus forming a single phase. FIG. 5 in Non Patent Literature 2 shows an optical microscope image of a film obtained by dissolving a blend of 50 parts by mass of methyl methacrylate resin and 50 parts by mass of polyvinyl butyral containing various amounts of vinyl alcohol units, in a solvent, and carrying out cast molding. This film has a phase separation structure in which the methyl methacrylate resins form various sizes of dispersed phases. Due to the use of a solvent in the production of this film, there are drawbacks such as low productivity and contamination of film production environments.    Non Patent Literature 2: Macromolecules, Vol. 34, 4277 (2001)
Non Patent Literature 3 teaches that polyvinyl butyral and polymethyl methacrylate having a weight average molecular weight of 120,000 were melt-kneaded in various proportions to obtain blends. According to Non Patent Literature 3, in blends containing larger amounts of polyvinyl butyral, elongation at fracture in a tension test is greater, yield behavior is observed, and toughness is improved. However, the blends containing larger amounts of polyvinyl butyral described in Non Patent Literature 3 had insufficient dynamic properties. In contrast, in blends containing polyvinyl butyral in an amount of less than 50% by mass, toughness was barely improved, and dynamic properties were insufficient.    Non Patent Literature 3: J. Ind. Eng. Chem., Vol. 8, No. 6, 530 (2002)
Patent Literature 6 discloses a resin composition composed of a plasticized polyvinyl acetal resin and a block copolymer containing a methacrylic polymer block and an acrylic polymer block. According to Patent Literature 6, this resin composition is used to bond two glass sheets together, and whitening phenomenon due to contact with air is suppressed. However, the resin composition has very low surface hardness.    Patent Literature 6: Japanese Patent Laid-Open No. 2003-40653